Process for transporting a multiphase system through a pipeline

ABSTRACT

The invention relates to a process for transporting a multiphase system through a pipeline. An oil-soluble polymer with a weight-average molecular weight (Mw) of at most 1,500 kg/mol has been added to the system. Preferably the process is applied for the transport of a natural gas-condensate system or for the transport of a petroleum-gas system. The polymer may be an olefin polymer which contains monomer units of ethylene and at least one alpha olefin or an olefin polymer which contains monomer units of ethylene, at lease one alpha olefin and at least one non-conjugated polyene.

The invention relates to a process for transporting a multiphase system,such as for example a gas-petroleum system or a natural gas-condensatesystem, through a pipeline. The invention also relates to the multiphasesystem

Said systems are released for example in the production of oil andnatural gas at sea, for example on the Continental Shelf of the NorthSea. Often the oil contains large quantities of so-called lightcomponents. Because the oil is under high pressure in the soil, thesecomponents remain in the liquid phase. During the recovery of the oilhowever these components evaporate as a result of the pressure reductionthat occurs. As a result a gas-petroleum system must be transported inthe pipelines from the recovery point to the production platform andfrom the production platform to the coast.

It also occurs that during the recovery of natural gas a condensate isformed of so-called heavy components in the natural gas, for examplebecause before its recovery the gas in the soil has a high temperatureand after the recovery cools off or because a condensate is formed dueto pressure reduction of the gas (also referred to as “retrogradecondensation”). Also in this case a multiphase system is transportedthen, now in the form of a natural gas-condensate system, from therecovery point to the production platform and from the productionplatform to the coast.

It is of course also possible that such systems are transported bypipelines overland, for example from a well to a processing plant wheregas and liquid are separated.

Said transport operations take place over great distances. Thus, thedistance between the well or the recovery point and the platform hasincreased to dozens of kilometres as a result of the advanced productiontechniques. The distance between the production platform and the coastcan easily increase to a hundred kilometers or even hundreds ofkilometers. Therefore the transport of the gas-petroleum and the naturalgas-condensate systems is a critical factor in the recovery of petroleumand natural gas. To increase the transport volume a higher pressure dropand/or lines with a larger diameter would have to be used. In both casesmuch larger investments in the installations are required.

It is the object of the invention to provide a process for increasingthe transport volume without the above-mentioned disadvantage.

This aim is achieved in that a multiphase system is transported to whichan oil-soluble polymer has been added with a weight-average molecularweight (Mw) of at most 1,500 kg/mol.

The multiphase system preferably comprises an organic liquid. Morepreferably the multiphase system is a gas-liquid system.

According to a preferred embodiment of the invention Mw is at most 800kg/mol.

According to a further preferred embodiment of the invention Mw is atmost 400 kg/mol.

Preferably Mw is at least 10 kg/mol and more preferably at least 300kg/mol.

By adding the polymer it becomes possible to transport a larger volumeof the multiphase system per unit of time through the pipeline, withoutit being necessary to increase the pressure drop across the pipeline orto choose a pipeline with a larger transport volume.

The way in which the multiphase system, such as for example thegas-liquis system, is present in the pipeline is for example dependenton the angle of the axis of the line to the horizontal plane and on theflowrates of the gas and the liquid, respectively, in the pipeline.Thus, in a pipeline of which the axis is at a relatively small angle tothe horizontal plane, a liquid layer will often be present with on theliquid layer a gas layer, which flow with different velocity through theline.

If there is more liquid in the line and if the gas velocity is highenough waves can be dormed that fill the entire pipe diameter. Theseliquid plugs are called slugs and these slugs give rise to a large lossin pressure during the transport.

Further it is possible that in the multiphase system solids are present,such as for example sand particles. It is also possible that besides theoil or the condensate a liquid aqueous phase or methanol is present.

Examples of suitable polymers to be used in the process according to theinvention if an organic multiphase system is used, such as for examplenatural gas-condensate or gas-petroleum, are oil-soluble polymers.Suitable oil-soluble polymers include for example polysiloxanes, naturalrubber, olefin polymers, and polystyrene. Ther preferred polymers areolefin polymers. Suitable olefin polymers are for example polypropylene,polybutene and copolymers of ethylene and an alpha olefin

Preferably the selected polymer is easily soluble in the liquid of themultiphase system.

Very good results are achieved if the selected polymer is a (first)olefin polymer containing monomer units of ethylene and at least onealpha olefin and/or if the selected polymer is a ( second) olefinpolymer containing monomer units of ethylene, at least one alpha olefinand at least one non-conjugated polyene.

The olefin polymers provide a highly improved transport through thepipeline and furthermore offer the advantage that they have a lowsensitivity to mechanical degradation (a high “shear stability”), theyare chemically inert, they cause no fouling in or blockage of thepipelines and they can easily be added to the multiphase system due totheir relatively low viscosity if they are added in a homogeneoussolution ( compared to ultra high molecular weight polymers).

As monomer units of an alpha olefin the olefin polymers may contain forexample monomer units of an alpha olefin of 3-23 carbon atoms. Suitableexamples include propylene, 1-butene, 1-pentene, 1-hexene and 1-octeneand styrene, monomer units of (alpha, omega) diolefins, such as forexample 1,5-hexadiene, 1,6-heptadiene and 1,7-octadiene, branched alphaolefins, such as for example 4-methyl-1-butene, 5-methyl-1-penteneand/or 6-methyl-1-heptene.

More preferably the monomer units of the alpha olefin monomer units arepropylene, 1-butene, 1-pentene, 1-hexene and 1-octene. Most preferablythe olefin polymer contains monomer units of propylene.

As monomer units of a non-conjugated polyene the (second) olefin polymercontains for example polyene (C), the molecule of which contains a C—Cdouble bond which is polymerisable with for example a Ziegler-Nattacatalyst, in addition to at least another C—C double bond and/or polyene(D), the molecule of which contains several C—C double bonds which arepolymerisable with a Ziegler-Natta catalyst.

Polyene (C) and polyene (D) can be both aliphatic and alicylic.Aliphatic polyenes generally contain 3 to 20 carbon atoms.

Suitable examples of polyene (C) include 1,4,8-decatrienes,1,4-hexadiene and/or 4-methyl-1,4-hexadiene.

Suitable examples of polyene (D) include 1,4-pentadiene, 1,5-hexadiene,1,7-octadiene and/or 1,4,9-decatriene.

Alicyclic polyenes, with or without a bridge group, may be both mono-and polycyclic.

Suitable examples of alicyclic polyenes (C) include alkylidenenorbonenes, in particular the 5-alkylidene-2-norbornenes, in which thealkylidene group contains 1 to 20, and preferably 1 to 8 carbon atoms,and some alkenyl norbornenes, in particular the 5-alkenyl-2-norbornenes,in which the alkenyl group contains 2 to 20 and preferably 2 to 10carbon atoms and in which the alkenyl group does not contain a terminaldouble C—C bond, for example 5-(2′-methyl-2′butenyl)-2-norbornene and5-(3′-methyl-2′butenyl)-2-norbornene.

Preferably polyene (C) is 5-ethylidene-2-norbornene.

Suitable examples of alicyclic polyenes (D) include vinylnorbornene,norbornadiene and the alkyl derivatives, and/or dicyclopentadiene.

Preferably polyene (D) is dicyclopentadiene, 5-vinyl-2-norbornene and/or1,5-hexadiene is applied.

The (first) olefin polymer contains for example 15-80 mole % monomerunits of ethylene and 20-85 mole % of the alpha olefin. Preferably thefirst olefin polymer contains 35-75 mole % monomer units of ethylene and25-65 mole % of the alpha olefin, still more preferably the olefinpolymer contains 50-70 mole % monomer units of ethylene and 50-30 mole %of the alpha olefin, most preferably the olefin polymer contains 55-65mole % monomer units of ethylene and 45-35 mole % of the alpha olefin.

Preferably the olefin polymer contains no or only a small quantity ofcrystallinity, so that an even better solubility in organic liquidsystems is achieved.

Preferably the olefin polymer has a crystallinity of at most 5%,measured with the aid of DSC (differential scanning calorimetry). Morepreferably the polymer has a crystallinity of at most 1%.

The crystallinity is calculated from the heat of crystallisation,derived per se from a DSC curve by heating up a polymer sample at a rateof 20° C./min to a temperature of 200° C., keeping it at thattemperature for 5 minutes and afterwards recording the curve while thesample is cooled off at a rate of 5° C./min. The crystallinity iscalculated from the heat of crystallisation being expressed in J/g withthe aid of the formula (I):% crystallinity=heat of crystallisation/2.94   (I)

According to a further preferred embodiment of the invention the(second) olefin polymer contains 0.1-10 mole % monomer units of thepolyene and the monomer units of ethylene and the alpha olefinpreferably having the molar ratio of the monomer units of ethylene andthe alpha olefin of the first olefin polymer. More preferably the(second) olefin polymer contains 0.2-8 mole %, more preferably 0.5-4mole %, monomer units of the polyene.

It is possible to use conventional polymerisation techniques for thepolymerisation of the olefin polymer, such as for example polymerisationin the presence of a Ziegler-Natta catalyst or an anionic catalyst. Itis also possible to polymerise in the presence of a metallocenecatalyst.

The polymerisation may be carried out for example in a solution, in aslurry or in the gas phase.

Preferably the polymerisation is a solution polymerisation. Examples ofsuitable solvents are linear paraffins with 5 to 8 carbon atoms.Preferably hexane is used as the solvent. It is also possible to usearomatic hydrocarbons as the solvent. Preferably aromatic hydrocarbonsare used with only one benzene nucleus, such as for example benzene andtoluene. It is also possible to use saturated cyclic hydrocarbons assolvent, preferably having a boiling point that lies in the the boilingpoint range of the above-mentioned linear paraffins.

If the polymerisation is a slurry polymerisation, preferably liquidpropylene is used as the liquid for the slurry.

In one preferred embodiment of the invention the olefinic polymercontains polar groups The polar groups may for example be part of themain chain, may be part of side groups on the chain and/or may be sidegroups. The presence of these polar groups results in reduced corrosionand in an improved dispersion of small solid particles.

Good results are achieved if the polar groups contain at least an oxygenatom and/or a nitrogen atom.

Preferably the olefinic polymer with polar groups is the above-mentionedolefin polymer being grafted with maleic acid, maleic anhydride and/oran ester of maleic acid. Preferably the grafted olefin polymer containsat least 0.01 mole, more preferably at least 0.05 mole, side groupsderived from maleic anhydride and/or an ester of maleic acid per 1000grams of polymer. Preferably the grafted olefin polymer contains at most0.5 mole side groups derived from maleic anhydride and/or an ester ofmaleic acid per 1000 grams of polymer.

The olefin polymer may be grafted with for example acrylic acid,itaconic anhydride, maleic anhydride, N-vinylpyrrolidone or acrylamide.

Preferably the olefin polymer is grafted with maleic anhydride or theesters of maleic acid with the aid of a free radical initiator, in asolution or in bulk.

Very good results are achieved if grafting is carried out in a solutionwith for example hexane as the solvent. This reaction is carried out ata temperature in the range of for example 100° C. to 250° C. andpreferably in the range of 120° C. to 190° C. and still more preferablyin the range between of 150° C. to 180° C.

Suitable free radical initiators include for example peroxides,hydroperoxides and azo compounds for example azobis-isobutyronitrile,dicumyl peroxide and/or 5-dimethylhex-3-yne-2,5-bis-tertiary-butylperoxide. The initiator may be used for example in a quantity of 0.005-2weight % (on the basis of the reaction mixture).

Preferably the grafting is carried out in the absence of oxygen.

If the olefin polymer is grafted in bulk, the maleic anhydride or themaleic acid ester and the free radical initiator is mixed, with a meltof the olefin polymer, for example in an extruder or a mixer. Thegrafting takes place for example at a temperature between 150° C. and300° C.

The process for grafting the olefin polymer may be carried out bothcontinuously and in a batch process. Preferably however the process forgrafting the olefin polymer is carried out in a continuous process.After grafting a subsequent modification, with for example an aminetowards an imide, is possible.

The polymers having the molecular weights as mentioned above may also beobtained by mechanical or thermal degradation of higher molecular weightpolymers

Good results are achieved if the polymer is added to the multiphasesystem in a quantity between for example 1 and 1000 ppm. Preferably thisquantity is between 5 and 500 ppm and more preferably this quantity isbetween 10 and 250 ppm.

For the addition to the multiphase system the polymer is preferablydissolved in a solvent suitable for the polymer.

The polymer thus dissolved can be added for example to the multiphasesystem and mixed with it with the aid of known equipment that is usedfor adding different additives to multiphase systems, such as forexample corrosion inhibitors and additives to counter hydrate formationin petroleum, for example methanol.

Very good results are achieved if as multiphase system, a naturalgas-condensate or a gas-petroleum system is transported with the aid ofthe process according to the invention. Most preferably the process isapplied to a natural gas-condensate system, because a very largereduction in pressuredrop is achieved in that case.

The process according to the invention is applied preferably for thetransport of a multiphase system in pipelines with a stratified flow, anannular flow or a slug flow. In the case of annular flow a cross-sectionof the pipe shows that the whole wall is covered with a liquid layer. Inthe case of stratified flow a cross-section of the pipe shows that partof the wall is covered with a liquid layer.

Most preferably the process according to the invention is applied with astratified flow.

U.S. Pat. No. 5,107,843 discloses the use of a water-soluble dragreducer in a water-oil-gas system. The water-soluble drag reducer is awater-soluble polymer containing polar groups. In contrast the presentinvention is directed to oil-soluble polymers.

EP-A-243127 discloses that a polymer may be added to oil to improve thetransport through pipelines. However in contrast to the presentinvention this process is not concerned with a multiphase system and isdirected to a pure liquid system, with consequently completely differenttransport mechanisms being involved. Further, EP-A-243127 teachesprecisely that a polymer with a very high molecular weight must be used,with an intrinsic viscosity of even more than 10 dl/g.

SU-A-(11)1361308 discloses that a copolymer of ethylene and propylenewith a molecular weight of 56000-60000 can be added to petroleum fortransport through pipelines. However in contrast to the presentinvention this process is not concerned with a multiphase system becauseit is directed to a heavy petroleum with a high resin content. Thepolymer is added as a waxinhibitor to counter the formation ofaggregates in paraffin-containing resin-like oils. In the multiphasesystems that are used in the process according to the present inventionit is precisely light oils that are concerned and precisely the problemof improving the transport volume is solved, while in SU-A (11)1361308the emphasis lies on lowering the pressure which is necessary to restartthe oil flow through the pipeline after an incident

U.S. Pat. No. 3,559,664 discloses a process for transporting liquidhydrocarbons, with the addition of a block copolymer of ethylene andpropylene to the hydrocarbons. However in contrast to the presentinvention this process is not concerned with a multiphase system becauseit is directed to a pure liquid system. Further a high-molecular weightpolymer is used with an intrinsic viscosity of more than 5 dl/g. Thesepolymers are not shear stable and these polymers cannot be used inconventional pumping systems which are used in the present field ofapplication.

The invention will now be elucidated by means of the followingnon-restrictive examples.

EXAMPLES I-III AND COMPARATIVE EXPERIMENTS A-C

The polymers used in the Examples I-III are:

-   -   EPM-1: an ethylene propylene copolymer having an ethylene        content of 58 mole % grafted with maleic anhydride and capped        with N-phenyl para phenylene diamine (NPPDA) and Mw 90,000 g/mol    -   EPM-2: an ethylene propylene copolymer having an ethylene        content of 58 mole % and Mw of 140,000 g/mol.    -   EPM-3: an ethylene propylene copolymer having an ethylene        content of 58 mole % and Mw of 90,000 g/mol

EXAMPLE I AND COMPARATIVE EXPERIMENT A

In a test rig CO₂ gas to which cyclopentane was added in differentratios was pumped through a pipeline to a storage tank at 25° C. atdifferent flowrates. The CO₂ cyclopentane system served as a model for anatural gas-condensate system.

In Example I, 65 ppm EPM-1 had been dissolved in the cyclopentanewhereas in Comparative Experiment A no polymer had been dissolved in thecyclopentane.

A steel pipe with a cross-section of 100 mm was used as pipeline. Thepressure drop over a section of 5 metres of the line was measured. Thetemperature amounted to 25° C.

The results are shown in the Table as a function of the superficialvelocity of cyclopentane (vsl) and the superficial velocity of the CO₂(vsg). Also shown are the pressure drop per metre of pipeline (ΔPex. 1,in Pa/m) measured in accordance with the example on the multiphasesystem to which the polymer had been added, and the pressure drop permetre (ΔPcomp. exp.A, in Pa/m) measured in accordance with theassociated comparative experiment on the corresponding multiphase systemwithout the polymer.

The effect of the presence of the polymer on the decrease in thepressure drop has been calculated according to the formula (II):Eff%=(ΔPcomp. exp A-ΔPex. 1)/ΔPcomp. exp A×100%   (II) TABLE 1 Comp.Exp. A Example I Vsl (m/s) Vsg (m/s) ΔPcomp. exp A (Pa/m) ΔPex. 1 (Pa/m)Eff. (%) 0.06 5.5 27 24  9 0.06 8 52 47 10 0.09 5.5 37 33  9 0.09 8 6357 11

Table 1 shows the influence of EPM-1 on pressure drop with CO₂cyclopentane system. It appears that due to the addition of the polymer(EPM-1) to the multiphase system a relative reduction in pressure dropof up to 11% occurs.

EXAMPLE II AND COMPARATIVE EXPERIMENT B

Example II and Comparative Experiment B were carried out as Example Iand Comparative Experiment A, but instead of EPM-1 EPM-2 was used at 50ppm concentration. The results are given in Table 2. TABLE 2 Comp. Exp.B ΔPcomp. exp A Example II Vsl (m/s) Vsg (m/s) (Pa/m) ΔPex. 2 (Pa/m)Eff. (%) 0.03 4 9 9 0 0.06 4 13 12 5 0.09 4 18 17 4 0.03 5.5 17 16 40.06 5.5 24 20 14 0.09 5.5 33 27 17 0.03 6.7 26 23 12 0.06 6.7 33 27 180.09 6.7 45 36 21 0.03 8 38 33 13 0.06 8 49 39 21 0.09 8 55 42 24

Table 2 shows the influence of EPM-2 on pressure drop with CO₂cyclopentane system It appears that by the addition of the polymer(EPM-2) to the multiphase system a relative reduction in pressure dropof up to 24% occurs.

EXAMPLE III AND COMPARATIVE EXPERIMENT C

Example III and Comparative Experiment C were carried out as Example Iand Comparative Experiment A, but instead of cyclopentane a lightpetroleum with a viscosity of 3 centipoise was used and instead ofEPM-1, EPM-3 was used at 50 ppm concentration The results are given inTable 3. TABLE 3 Comp. Exp. C Example III Vsl (m/s) Vsg (m/s) ΔPcomp.exp C (Pa/m) ΔPex. 3 (Pa/m) Eff. (%) 0.03 4 23 23 0 0.06 4 27 27 0 0.094 33 33 0 0.03 5.5 28 28 0 0.06 5.5 33 32 3 0.09 5.5 38 36 4 0.03 6.7 3837 4 0.06 6.7 43 40 6 0.09 6.7 49 46 7 0.03 8 48 46 5 0.06 8 54 50 80.09 8 59 54 8

Table 3 shows the influence of EPM-3 on pressure drop CO₂ petroleumsystem It appears that by the addition of the polymer (EPM-3) to themultiphase system a relative reduction in pressure drop of up to 8%occurs.

1-14. (canceled)
 15. A process for transporting a multiphase systemthrough a pipeline, wherein a multiphase system is transported to whichan oil-soluble polymer has been added with a weight-average molecularweight (Mw) of at most 400 kg/mol.
 16. A process according to claim 15,wherein the polymer is an olefin polymer which contains monomer units ofethylene and at least one alpha olefin or the polymer is an olefinpolymer which contains monomer units of ethylene, at least one alphaolefin and at least one non-conjugated polyene.
 17. A process accordingto claim 15, wherein the alpha olefin is propylene.
 18. A processaccording to claim 16, wherein the olefin polymer contains 15-80 mole %monomer units of ethylene and 20-85 mole % monomer units of an alphaolefin or the olefin polymer contains monomer units of ethylene, unitsof an alpha olefin and 0.1-10 mole % monomer units of the polyenewherein the monomer units of ethylene and the alpha olefin have the samemolar ratio as in said (first) olefin polymer.
 19. A process accordingto claim 18, wherein the olefin polymer contains 35-75 mole % monomerunits of ethylene and 25-65 mole % monomer units of an alpha olefin. 20.A process according to claim 16, wherein the olefin polymer has acrystallinity of at most 5%.
 21. A process according to claim 16,wherein the olefin polymer contains polar groups.
 22. A processaccording to claim 21, wherein the olefin polymer containing polargroups is an olefin polymer grafted with maleic anhydride and/or anester of maleic acid.
 23. A process according to claim 15, wherein themultiphase system is a gas petroleum system.
 24. A process according toclaim 15, wherein the multiphase system is a natural gas-condensatesystem.
 25. A process according to claim 15, wherein the multiphasesystem has a stratified, an annular flow or a slug flow.
 26. Amultiphase system wherein the multiphased system an oil-soluble polymerhas been added with a weight-average molecular weight (Mw) of at most400 kg/kmol.